1. Field of the Invention
The present invention relates to a shield panel having defogging function using electric heating, and the invention also relates to a helmet having such a shield panel.
2. Description of the Related Art
A helmet that a driver or a passenger of an autobicycle, an airplane, a snowmobile or the like wears is provided with a shield panel which is also called a visor for preventing wind, a flying object or the like from hitting his or her face. When the autobicycle or the like runs at high speed, especially in a cold district, however, since the shield panel is air-cooled, water vapor in exhalation of the wearer impinges on the cold shield panel to cause condensation and freezing thereon, which impairs wearer""s visibility. Thereupon, in order to prevent condensation or freezing to secure the wearer""s visibility, there is conceived a shield panel having defogging function using electric heating.
A conventional shield panel has a transparent panel provided with a transparent electrically conductive film having uniform thickness, and the transparent panel is provided at its upper and lower end portions with a pair of electrodes extending on the transparent electrically conductive film. The transparent electrically conductive film is a thin film made of indium tin oxide (ITO), gold or the like. Each of the electrodes is formed of conductive paint or the like applied by screen printing. The electrode is provided at its portion with a terminal, and the electrode is connected to a power source of the autobicycle or the like through the terminal, conductive wire and the like. When a voltage is applied to the transparent electrically conductive film and a current flows therethrough, the transparent electrically conductive film heats and the transparent panel is heated by the transparent electrically conductive film (e.g., Japanese Utility Model Publication No. H2-7843).
However, the conductive paint comprises insulative high polymer resin-based paint into which metal particles or metal fibers such as silver are mixed, and it is not possible to bring the metal particles into sufficient contact with each other or the metal fibers into sufficient contact with each other. Therefore, the resistance of the conductive paint cannot be sufficiently lowered. If mixing ratio of metal is increased, the resistance of the conductive paint can be lowered. However, it becomes difficult to apply the conductive paint to the transparent electrically conductive film, adhesion of the conductive paint toward the transparent electrically conductive film is also lowered, and this is not practical.
Therefore, in the conventional shield panel, the resistance of the electrode is high, and if the terminal for the electrode and an end portion of the electrode are separated from each other, potential drop is caused in the electrode, and a voltage applied between the pair of electrodes is not effectively applied to the transparent electrically conductive film of the transparent panel. A voltage of a power source of an autobicycle is as low as 12V, but a voltage applied to the transparent electrically conductive film is further lower, the transparent panel is not effectively heated by the transparent electrically conductive film, and the defogging effect of the conventional shield panel is not always effective. Although it is possible to increase a voltage obtained from the power source, since there is a risk of an electric shock accident is high in view of the usage pattern, it is not preferable.
In the conventional shield panel, the conductive paint is applied by screen printing to form the electrode. Since one time screen printing can only apply the paint of about 25 xcexcm thickness, in order to form a thick electrode to lower the resistance of the electrode, it is necessary to repeat the application many times. Further, in the case of screen painting, it is easy to apply the paint on a flat surface, but it is not easy to apply the paint on a curved surface. For these reasons, in the conventional shield panel, it is not possible to easily form the electrode on the transparent electrically conductive film of the transparent panel, and manufacturing cost is high.
Further, a heating value of the transparent electrically conductive film is inversely proportional to a resistance of the transparent electrically conductive film if a voltage applied to the transparent electrically conductive film is constant. On the other hand, in order to secure the wearer""s visibility, a width of the transparent panel is generally set such that the width at a central portion in the lateral direction is wider than that at both end portions in the lateral direction. Therefore, a distance between the pair of electrodes provided at upper and lower end portions of the transparent panel is also wider at the central portion than at the both end portions. Since the thickness of the transparent electrically conductive film is uniform, the resistance of the transparent electrically conductive film at the central portion is higher than that at the both end portions. The heating value of the transparent electrically conductive film at the central portion is smaller than that of the both end portions.
However, the central portion in the lateral direction of the shield panel is air-cooled strongly by running of the autobicycle or the like, but the both end portions in the lateral direction are air-cooled weakly as compared with the central portion, and exhalation of the wearer impinges mainly on the central portion in the lateral direction of the transparent panel. As a result, water vapor in exhalation of the wearer is prone to generate the condensation or freezing on the shield panel, and with this reason also, the defogging effect of the conventional shield panel is not always effective.
It is an object of the present invention to provide a shield panel having an effective defogging effect, and to provide a helmet having such a shield panel.
In a shield panel according to the present invention, a pair of electrodes for a transparent electrically conductive film provided on a transparent panel have metal foils. Each of the metal foils is a homogeneous continuous phase made of metal only and thus, the resistance is low. Further, the metal foils extend along the transparent electrically conductive film while electrically connected to the transparent electrically conductive film. For these reasons, even if a terminal for the electrode and an end portion of the electrode are separated from each other, the resistance of the electrode is low, and potential drop is not caused almost at all in the electrode. As a result, since a voltage applied between the pair of electrodes is effectively applied to the transparent electrically conductive film of the transparent panel, even if a power source voltage is not high, the transparent panel is effectively heated, and a defogging effect is effective. Further, since the terminals for the electrodes need not be located at central portions of the electrodes, a degree of freedom in design is high.
In a preferred shield panel according to the present invention, the metal foil is adhered to the transparent electrically conductive film through a conductive adhesive layer. Therefore, even if the transparent electrically conductive film of the transparent panel is three-dimensional curved surface, it is possible to easily mount the electrode to the transparent electrically conductive film, and thus the manufacturing cost is low. Further, a difference in thermal expansion between the metal foil and the transparent electrically conductive film or the transparent panel is reduced by the conductive adhesive layer, the electrodes are less prone to be peeled off from the transparent electrically conductive film, and thus the reliability is high.
In a preferred shield panel according to the present invention, a ratio of the thickness of the transparent electrically conductive film at the central portion in the lateral direction of the transparent panel to the thickness at the both end portions is 1.1 or greater. Therefore, if a ratio of a distance between the pair of electrodes at the central portion to the distance at the both end portions is about 1.1, the resistance of the transparent electrically conductive film at the central portion is equal to or smaller than the resistance at the both end portions, and the transparent electrically conductive film at the central portion heats to a degree equal to or higher than that at the both end portions. Even if the above ratio of distance exceeds 1.1, the heating value of the transparent electrically conductive film at the central portion is secured. Therefore, though the central portion of the transparent panel is air-cooled stronger than the both end portions and the exhalation of the wearer impinges on the central portion when the shield panel is used, a defogging effect is effective. However, if the ratio of the thickness is smaller than 1.1, the heating value of the transparent electrically conductive film at the central portion is not secured.
In a preferred shield panel according to the present invention, the ratio of the thickness of the transparent electrically conductive film at the central portion in the lateral direction of the transparent panel to the thickness at the both end portions is equal to or greater than the ratio of a distance between the pair of electrodes at the central portion to the distance at the both end portions. Therefore, even if the width of the transparent panel is wider at the central portion than at the both end portions to secure the wearer""s visibility, the resistance of the transparent electrically conductive film at the central portion is equal to or smaller than that at the both end portions. As a result, potential drop is not caused almost at all in the electrode, and even if the voltage between the pair of electrodes is substantially equal in the central portion and the both end portions, the transparent electrically conductive film at the central portion heats to a degree equal to or higher than that at the both end portions. Therefore, though the central portion of the transparent panel is air-cooled stronger than the both end portions and the exhalation impinges on the central portion when the shield panel is used, a defogging effect is effective.
In a preferred shield panel according to the present invention, a space is formed between the pair of transparent panels, and the transparent electrically conductive film is provided on the inner transparent panel. Therefore, the inner side transparent panel is heated by the transparent electrically conductive film, and the space between the pair of transparent panels functions as a heat insulating layer toward the inner transparent panel. Thus, even if a power source voltage is not high, the inner transparent panel is effectively heated, and a defogging effect is effective.
In a preferred shield panel according to the present invention, since a thickness of the metal foil is 10 xcexcm or greater, the resistance of the electrode is low, but since the thickness of the metal foil is 100 xcexcm or smaller, the flexibility of the electrode is high, and the electrode can be easily mounted to a transparent electrically conductive film having a three-dimensional curved surface. Therefore, the manufacturing cost is low.
In a preferred shield panel according to the present invention, the metal foil is a copper foil, and the copper has a low resistance. Therefore, even if the terminal for the electrode and the end portion of the electrode are separated from each other, the resistance of the electrode is low, and potential drop is less prone to be caused in the electrode, and a voltage applied between the pair of electrodes is effectively applied to the transparent electrically conductive film of the transparent panel. Therefore, even if a power source voltage is not high, the transparent panel is further effectively heated, and a defogging effect is further effective.
In a further preferred shield panel according to the present invention, since a thickness of the conductive adhesive layer is 10 xcexcm or greater, the metal foil is reliably adhered to the transparent electrically conductive film, and the electrodes are less prone to be peeled off from the transparent electrically conductive film. Further, since the thickness of the conductive adhesive layer is 50 xcexcm or smaller, the heat is easily transmitted between the transparent electrically conductive film and the metal foil, and the heat can be easily transmitted between regions of the transparent electrically conductive film through the metal foil. Therefore, even if a current flows through the transparent electrically conductive film unevenly due to non-uniform composition in the conductive adhesive layer, the transparent electrically conductive film and the conductive adhesive layer are prevented from local heating, and the transparent electrically conductive film and the conductive adhesive layer are less prone to be deteriorated. Therefore, the reliability is high and its life is long.
In a helmet according to the present invention, a pair of electrodes for a transparent electrically conductive film provided in a shield panel have metal foils. Each of the metal foils is a homogeneous continuous phase made of metal only, and thus the resistance is low. Further, the metal foils extend along the transparent electrically conductive film while electrically connected to the transparent electrically conductive film. For these reasons, even if a terminal for the electrode and an end portion of the electrode are separated from each other, the resistance of the electrode is low, and potential drop is not caused almost at all in the electrode. As a result, since a voltage applied between the pair of electrodes is effectively applied to the transparent electrically conductive film of the shield panel, even if a power source voltage is not high, the shield panel is effectively heated, and a defogging effect of the shield panel is effective. Further, since the terminals for the electrodes need not be located at central portions of the electrodes, a degree of freedom in design of the shield panel is high.
In a preferred helmet according to the present invention, there is provided with a mask which, by covering a nose and a mouth of a wearer, forms a space in communication with an outside in a state in which the space is isolated from a space facing an inner surface of the shield panel. For this reason, exhalation of the wearer is discharged to the outside and does not impinge on the shield panel. Therefore, in cooperation with circumstances that even if the power source voltage is not high the shield panel is effectively heated, the defogging effect of the shield panel is further effective.
In a preferred helmet according to the present invention, there is provided with an exhalation guide plate which is opposed to the nose of the wearer for guiding exhalation of the wearer to an outside. For this reason, exhalation of the wearer is easily discharged to the outside and does not easily impinge on the shield panel. Therefore, in cooperation with circumstances that even if the power source voltage is not high the shield panel is effectively heated, the defogging effect of the shield panel is further effective.